Why they changed it... I have no idea. But the change only matters to people with new Model 98s. All of the old Model 98s will still accept the new barrels, but the new ones with the changed threads will only be able to accept the new barrels. So if you have an old one, you don't have to worry.

Here is the quote form Tippmann:

"We changed the ID and OD of the receiver and the barrel threads of 98 Customs about two weeks ago. All guns with serial numbers above 495,501 will not accept the old style barrels. All barrels being made now, have the new style thread. However, all new barrels will also fit older guns regardless of serial number.
If you have an aftermarket barrel with the older style threads, it will not fit the new receiver. We suggest that you contact your supplier for a new style barrel that will fit. All barrel manufacturers were notified of the change long before we ever made any changes."